1. Field of the Invention
The present invention relates to a power transmission system for transmitting power between an engine and a load.
2. Description of the Related Art
Power transmission systems of this type are disclosed in Japanese Translated Publications for PCT No. 2002-513118 (hereinafter referred to as patent document 1) and No. 2002-543340 (patent document 2), and Shuiwen Shen, Alex Serrarens, Maarten Steinbuch, Frans Veldpaus, “Coordinated Control of a Mechanical Hybrid Driveline with a Continuously Variable Transmission”, JSAE Review 22, 2001, pp. 453-461 (non-patent document 1). According to these documents, a planetary gear mechanism is provided in parallel to a continuously variable speed variator which transmits power of an engine to a load while changing speed, and the input/output shaft of the continuously variable speed variator and a flywheel are coupled to separate rotating elements of the planetary gear mechanism. When a speed changing operation is performed by the speed variator, power is transmitted between the engine and the flywheel. For example, when downshifting, rotational energy of the flywheel is discharged for use in increasing rotation of the engine. On the other hand, when upshifting, power of the engine is absorbed by the flywheel. As such, according to the above-noted documents, rotational energy of the flywheel is used to assist the speed changing operations of the continuously variable speed variator.
Furthermore, power transmission systems are disclosed in Japanese Translated Publication for PCT No. Hei 11-504415 (patent document 3) and Japanese Patent Laid-Open Publications No. 2002-48213 (patent document 4), and No. 2003-247623 (patent document 5).
According to the above-referenced patent documents 1 and 2 and non-patent document 1, discharge or absorption of rotational energy by the flywheel is passively carried out when the gear ratio of the speed variator is changed, while power transmission to the planetary gear mechanism is also performed passively. For this reason, it is not possible to actively control the power transmitted to the planetary gear mechanism in a manner independent from the speed changing control of the speed variator. Further, patent documents 1 and 2 and non-patent document 1 make absolutely no reference to the concept of actively controlling distribution of power transmitted to the speed variator and power transmitted to the planetary gear mechanism. As such, according to systems described in patent documents 1 and 2 and non-patent document 1, it is difficult to appropriately control distribution of power transmitted to the speed variator and power transmitted to the planetary gear mechanism, making it difficult to achieve reduction of speed variator capacity and enhancement of power transmission efficiency.